High recording density has become an indispensable requirement to the performance of magnetic recording media, thereby requiring a decrease in the thickness of the magnetic layer in the media. However, the decrease of the magnetic layer thickness (about 1 .mu.m or less) has been accompanied by attendant practical problems; e.g., the durability of a magnetic layer becomes noticeably decreased at low thicknesses. Many techniques of providing an undercoat layer between a magnetic layer and a base film have been examined to improve the durability of such a magnetic layer.
Japanese Unexamined Patent Publication No. 1-213822 discloses that in a magnetic recording medium provided with an undercoat layer containing a non-magnetic iron oxide and a carbon black, electroconductivity and the surface smoothness of a magnetic layer becomes improved and in turn, the electromagnetic transfer characteristic or durability is improved. However, this reference makes no mention of an excellent effect attributable to the acicular shape of non-magnetic iron oxide particles.
In addition, Japanese Patent Application No. 1-300419 discloses that the electroconductivity of a recording medium, and the surface smoothness and electromagnetic transfer characteristic of the magnetic recording medium are improved by the use of acicular non-magnetic particles as an insulator for an undercoat layer containing a carbon black. Although the patent application includes a description to the effect that acicular non-magnetic iron oxide can be used as acicular non-magnetic particles, this acicular non-magnetic iron oxide is expected for its function as an insulator. Accordingly, the amount of the non-magnetic iron oxide used based on the amount of the carbon black is small (20 to 60% by volume ratio; about 40 to about 80% converted to weight ratio). However, when the proportion of the non-magnetic iron oxide is so low as mentioned above, there cannot be obtained such high surface smoothness as obtained by the present invention.
On the other hand, it is known that a carbon black having a structure constitution has high electroconductivity. Japanese Unexamined Patent Publication No. 61-177631 discloses a technique of controlling the electrostatic property of a magnetic layer by the utilization of a carbon black having a structured constitution. In this patent publication, the use of a carbon black with an oil absorbency of 90 (DBP) ml/100 g or more is recommended; however this oil absorbency is insufficient according to the present invention.